Rosenberg.Miocene.Rockies.erosion2014 .pdf

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In the Colorado Rocky Mountains, the
association of high topography and low seismic velocity in the underlying mantle suggests
that recent changes in lithospheric buoyancy
may have been associated with surface uplift
of the range. This paper examines the relationships among late Cenozoic fluvial incision, channel steepness, and mantle velocity
domains along the western slope of the northern Colorado Rockies. New 40Ar/ 39Ar ages on
basalts capping the Tertiary Browns Park
Formation range from ca. 11 to 6 Ma and
provide markers from which we reconstruct
incision along the White, Yampa, and Little
Snake rivers. The magnitude of post–10 Ma
incision varies systematically from north
to south, increasing from ~500 m along the
Little Snake River to ~1500 m along the Colorado River. Spatial variations in the amount
of late Cenozoic incision are matched by metrics of channel steepness; the upper Colorado
River and its tributaries (e.g., Gunnison and
Dolores rivers) are two to three times steeper
than the Yampa and White rivers, and these
variations are independent of both discharge
and lithologic substrate. The coincidence of
steep river profiles with deep incision suggests that the fluvial systems are dynamically adjusting to an external forcing but is
not readily explained by a putative increase
in erosivity associated with late Cenozoic climate change. Rather, channel steepness correlates with the position of the channels relative to low-velocity mantle. We suggest that
the history of late Miocene–present incision
and channel adjustment reflects long-wavelength tilting across the western slope of the
Rocky Mountains.

One of the outstanding tectonic questions in
western North America regards the development and support of high topography (Fig. 1). It
has long been recognized that correlations exist
among high topography (Gregory and Chase,
1994), low-seismic-velocity mantle (Grand,
1994; Schmandt and Humphreys, 2010), high
heat flow (Sass et al., 1971; Reiter, 2008), relatively thin crust (Sheehan et al., 1995; Hansen
et al., 2013), and extrusive volcanism (Larson
et al., 1975; Kunk et al., 2002). Although these
data point to a role for mantle buoyancy in support of high topography, questions remain about
when and how such buoyancy was established.
A variety of potential mechanisms have been
proposed, including: hydration of lithospheric
mantle (Humphreys et al., 2003) and/or thermal re-equilibration following removal of the
Laramide slab (Roy et al., 2004, 2009), delamination and/or removal of lithospheric mantle
(Elkins-Tanton, 2005; Levander et al., 2011),
and changes in the mantle flow field (Moucha
et al., 2008; van Wijk et al., 2010; Forte et al.,
2010; Liu and Gurnis, 2010).
Recent geophysical studies focused on the
Colorado Rockies (Aster et al., 2009; Schmandt
and Humphreys, 2010) reveal a prominent
region of anomalously slow P- and S-wave
speeds (Coblentz et al., 2011; Karlstrom et al.,
2012) that resides in the upper mantle beneath
the region of highest topography (Fig. 2). This
observation reaffirms previous conclusions that
support high topography in Colorado largely
residing in the upper mantle (Grand, 1994; Sheehan et al., 1995). In fact, the Colorado Rockies exhibit some of the thinnest crust along the
range, and a negative correlation between crustal
thickness and high topography also favors man-

tle support for high topography (Hansen et al.,
2013). The timing of when this buoyancy was
established, however, is not known directly.
The timing and patterns of incision along fluvial systems within and adjacent to the Rocky
Mountains suggest a possible role for differential uplift of the range relative to the Colorado Plateau and Great Plains. In the northern
Colorado Rockies, the onset of fluvial incision
appears to coincide with the cessation of late
Tertiary deposition in intermontane basins (Larson et al., 1975; Buffler, 2003; McMillan et al.,
2006). Along the eastern flank of the range,
incision postdates deposition of the ca. 18–6 Ma
Ogallala Formation (McMillan et al., 2002,
2006). Notably, reconstruction of paleo-transport gradients (McMillan et al., 2002; Duller
et al., 2012) in these deposits argues for longwavelength tilting in excess of that expected
for a simple isostatic response to exhumation
(e.g., Leonard, 2002). Thus, some conclude that
tilting must have been, in part, driven by surface uplift within the Rockies (McMillan et al.,
2002; Riihimaki et al., 2007; Duller et al., 2012;
Nereson et al., 2013), but others argue that most,
if not all, recent incision may reflect climatically
modulated changes in erosive efficiency (e.g.,
Anderson et al., 2006; Wobus et al., 2010).
Along the western slope of the range, fluvial incision also appears to have initiated in
the past ca. 10 Ma (Kunk et al., 2002; Aslan
et al., 2008; Berlin, 2009; Aslan et al., 2010;
Karlstrom et al., 2012), but the mechanisms
driving incision remain uncertain. In particular,
the possibility that incision along the western
slope reflects upstream migration of a wave of
transient incision in response to drainage integration along the Colorado and Green rivers
(e.g., Pederson et al., 2002, 2013) presents an
additional complication. In an effort to deter-

TIMING AND MAGNITUDE OF
INCISION ALONG THE WESTERN
SLOPE OF THE COLORADO ROCKIES

35ºN

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Background: Previous Work on
Late Cenozoic Incision
Colorado River System
Much of the evidence for late Cenozoic tectonism in the Rocky Mountains relies on the
history of incision along major drainages (e.g.,
McMillan et al., 2006; Riihimaki et al., 2007).
An extensive body of work over the past two
decades indicates that the Colorado River has
incised ~1100–1500 m across the western slope
of the Rockies during the past 10 Ma (e.g., Larson et al., 1975; Kunk et al., 2002; Aslan et al.,
2010). We briefly summarize these constraints
below; relevant data are compiled in Table 1
and shown for reference on Figure 3. Following
Kunk et al. (2002), we exclude sites from within
regions known to have experienced collapse
during evaporite dissolution.
Most of the key markers used to reconstruct
fluvial incision along the main stem of the
Colorado River rely on associations of fluvial
gravels representing ancestral river deposits
with basalt flows (Table 1). The westernmost of
these is located at Grand Mesa, just upstream
from Grand Junction, Colorado (Fig. 3), where
the basal basalt flow (10.8 ± 0.2 Ma; Kunk
et al., 2002) overlies river gravels at ~1500 m
above the present-day river (Aslan et al., 2010).

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mine whether late Tertiary incision along the
western slope reflects differential rock uplift
associated with changes in mantle buoyancy
(Aslan et al., 2010; Darling et al., 2012; Karlstrom et al., 2012), we examine the White,
Yampa, and Little Snake rivers in Colorado
(Fig. 2). Recent analyses of the regional patterns
of channel steepness (ksn, a measure of channel
slope normalized for contributing drainage area;
Kirby and Whipple, 2012) reveal spatial differences that appear to correspond to the position
of rivers relative to low-velocity mantle beneath
the range (Karlstrom et al., 2012) and do not
reflect spatial differences in discharge (Pederson and Tressler, 2012). In this paper we present
a detailed analysis of river longitudinal profiles
and their relationship to substrate lithology and
combine this analysis with new 40Ar/39Ar ages
of late Cenozoic basalts that provide new constraints on the timing and magnitude of fluvial
incision. Collectively, these observations reveal
spatial patterns in both channel steepness and in
the magnitude of post–10 Ma incision that help
deconvolve the relative roles of climate change,
drainage integration, and/or differential rock
uplift along the western flank of the Rockies.

100

200

Kilometers

Gila R

0

4400
Elevation (m)

Figure 1. Topography, physiographic provinces, and major rivers of the western United
States. Physiographic provinces shown by white dashed lines. Large black inset shows the
study area, and smaller insets outline the areas of Figures 3 and 4.

Farther upstream, the Colorado River flows
between Battlement Mesa and Mount Callahan (Fig. 3). Here scattered basalt boulders on
the southern flank of Mount Callahan overlie
ancestral Colorado River gravels at ~1100 m
above the modern river (Berlin, 2009). Boulders from the deposit are similar in age (ca.
9.17 Ma; Berlin, 2009) to flows on Battlement
Mesa (ca. 9.3 Ma; Berlin, 2009) and are interpreted to represent debris-flow deposits derived
from these units and shed northward into the
ancient Colorado River valley (Berlin, 2009).
Because these deposits have been transported
across the axis of the canyon, ~1100 m represents a minimum value of incision (Berlin,
2009). The average modern transport slopes

Geosphere, August 2014

of debris-flow fans along the northern flank of
Battlement Mesa (~0.07; Berlin, 2009) and the
distance from Mount Callahan to the presentday position of the Colorado River (~4–5 km)
imply that there may have been up to ~280–
350 m of additional relief. Thus, it seems likely
that incision in the vicinity of Mount Callahan
and Battlement Mesa is in the range of ~1380–
1450 m. This value is consistent with estimates
(~1400–1500 m) derived from the projection of
Tertiary strata across the canyon from Battlement Mesa to the Roan Plateau (e.g., Bostick
and Freeman, 1984). Collectively, these observations imply that an ancestral Colorado River
was established across the western slope of the
Rockies by ca. 10 Ma (e.g., Aslan et al., 2010)

Gre
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Miocene evolution of Rocky Mountain topography

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δVp Velocity Anomaly at 100 km depth (%)

Elevation (m)
1000

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BC

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4000+

–2

–1

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1

2

Figure 2. Modern topography of the Rocky Mountain physiographic province and approximate extent of Tertiary
basins (left panel) and differential P-wave velocity at 100 km depth (right panel). Isolines on the right panel correspond to 0.5% of differential P-wave velocity. Geographic points for reference: GJ—Grand Junction, Colorado; R—Rifle, Colorado; SB—Steamboat Springs, Colorado; NP—North Park; SP—South Park; GM—Grand
Mesa; BC—Book Cliffs; FT—Flat Tops; SWB—Sand Wash Basin; UB—Uinta Basin; PB—Piceance Basin. Tomographic data from Schmandt and Humphreys (2010).
and that the river has subsequently incised
~1400–1500 m since that time.
Upstream of Glenwood Canyon (Fig. 3),
extensive preservation of ca. 10 Ma basalt flows
at similar elevations (3000–3400 m) along
the Colorado River suggests the presence of a
broad, low-relief erosional and/or transport surface prior to ca. 10 Ma (e.g., Larson et al., 1975;
Kunk et al., 2002). Incision into this surface was
probably ongoing by ca. 8 Ma, as suggested
by relationships between basalt flows and fluvial gravel at Spruce Ridge and Little Grand
Mesa (Kunk et al., 2002). Moreover, Kunk et al.
(2002) suggest that the presence of a young,
3.03 ± 0.02 Ma, high-elevation basalt at Gobbler’s Knob (Fig. 3), ~730 m above the modern
Colorado River, records an increase in the rate
of incision during the past ca. 3 Ma. However,
the base of the basalt flow at Gobbler’s Knob
is unexposed and is not known to be associated
with river gravels (Kunk et al., 2002). Thus,
the flow may have been emplaced significantly
above the ancestral Colorado River ca. 3 Ma
and may not directly constrain incision (Aslan
et al., 2010). Irrespective of this debate over the
pace of incision through time, it is clear that
incision in the upper Colorado River near Glen-

Green River System
In contrast to the reasonably well understood history of incision along the upper Colorado River, relatively little is known regarding
the timing and magnitude of incision along
the western slope of the Rockies in northern
Colorado. Here, the White, Yampa, and Little
Snake rivers are not entrenched in narrow canyons for long reaches, and deposits of ancient
fluvial gravels are exceedingly rare. However,
the region was the locus of sediment accumulation during Oligocene through Miocene
time (Kucera, 1962; Buffler, 1967; Izett, 1975;
Larson et al., 1975; Buffler, 2003; McMillan
et al., 2006), and these deposits, collectively
referred to as the Browns Park Formation
(Fig. 4) (originally described by Powell, 1876
and summarized by Kucera, 1962 and Buffler,
2003), have been deeply incised and eroded by
the modern drainage system. Thus, the degree
of preservation of basin sediments allows for a
minimum estimate of both the timing and magnitude of mass removed by fluvial activity (e.g.,
McMillan et al., 2006).
Regionally, the Browns Park Formation is
exposed in the Elkhead Mountains in the northeast, the Flat Tops in the south, and along the

Browns Park graben in the west (Fig. 4). There
are two, informally defined, members of the
Browns Park Formation—a lower basal conglomerate that rests unconformably on older
strata and an upper sandstone (Buffler, 2003).
The basal conglomerate is generally thin
(&lt;100 m) but thickens and becomes coarser
grained toward the margins of the basin; this
unit is interpreted to represent alluvial fans
being shed westward from the Park and Sierra
Madre ranges toward the Sand Wash Basin
(Buffler, 2003) and may be correlative with
deposits elsewhere referred to as the Bishop
Conglomerate (Boraas and Aslan, 2013). The
upper sandstone of the Browns Park Formation,
in contrast, ranges up to ~670 m thick and consists of sandstones of both eolian and alluvial
origin (Buffler, 2003). Paleocurrent indicators
in these sandstones suggest transport directions
toward the NNE (Buffler, 1967, 2003).
The age range of the Browns Park Formation
is relatively well known from intercalated tuffaceous deposits; these range from ca. 24.8 Ma
near the base of the sandstone member to ca.
8.2 Ma near the top of present exposure (Izett,
1975; Luft, 1985; summarized by Buffler,
2003). At City Mountain (Fig. 4, Locality 1), a
latite porphyry intruding the formation has been
dated to 7.6 ± 0.4 Ma (Buffler, 1967). Additionally, a volcanic tuff near the top of the Browns
Parks along Vermillion Creek (Fig. 4, Locality 2) has been dated at 9.8 ± 0.4 Ma (Naeser
et al., 1980). Collectively, these data suggest
that sediment accumulation in the region continued from ca. 24 to 8 Ma.
Of particular relevance to this study are basalt
flows that cap mesas and buttes throughout
the region and often overlie thick sections of
Browns Park Formation (~400–600 m; Buffler,
1967, 2003). The age of the uppermost Browns
Park Formation is similar to the flows themselves (K-Ar ages ranging from 9.5 ± 0.5 Ma
to 10.7 ± 0.5 Ma; Buffler, 1967, 2003). Because
these flows overlie the Browns Park Formation,
they are broadly consistent with a minimum
age for the formation of ca. 8–10 Ma (Buffler,
2003). Field relationships suggest, however, that
local relief generation during fluvial incision
likely postdates basalt emplacement, and thus
we pursued refined chronology from selected
localities in the region.
New Constraints on Late Miocene Incision
in Northern Colorado
In order to refine our understanding of the
switch from deposition of the Browns Park
Formation to incision along modern rivers,
we supplement existing chronology with new
40
Ar/39Ar ages from basalt flows. Localities

Figure 3. Simplified geologic map showing locations of previously dated markers, which provide constraints
on the timing and magnitude of incision along the Colorado River (modified from Tweto, 1979; Green, 1992).
The locations of evaporite collapse centers along the Colorado River (from Kunk et al., 2002) are outlined in
white. Data for previously published incision markers along the Colorado River are given in Table 1.
were carefully chosen where local relationships between the timing of deposition and
emplacement between volcanic units allowed
us to reconstruct the magnitude of incision
along primary rivers or their tributaries. Generally, these localities are characterized by basalt
flows that cap mesas and represent a formerly
continuous flow or sequence of flows that has
been dissected by incision along modern rivers
(Fig. 5). In a few cases, where flows are absent,
we use the exposed thickness of the Browns
Park Formation where the uppermost strata are
well dated by interbedded tuffs or intrusions
that place bounds on the position of the ancestral land surface. Because ancestral river gravels
are not preserved in these localities, our results
do not constitute a measure of fluvial incision
sensu stricto (Burbank and Anderson, 2011).

Rather, they provide conservative estimates for
the amount of relief generated in the landscape
during fluvial incision.
The region has experienced extensional faulting in late Miocene time (e.g., Kucera, 1962;
Buffler, 1967). Although fault slip is generally
limited to a few hundred meters, displacement
could have led to disruption of formerly continuous basalt flows. Therefore, we confine our
analysis to markers of incision within a given
fault block. At each locality, we compare our
results to the local thickness of preserved Miocene basin-fill sediments (Table 2). Because the
upper member of the Browns Park Formation
is typically subhorizontal, the exposed vertical thickness of the Browns Park Formation
provides a minimum bound on fluvial incision.
Our analyses utilize U.S. Geological Survey

Geosphere, August 2014

(USGS) 1° × 2° quadrangles (Tweto, 1976), the
geologic map of Wyoming (Love and Christiansen, 1985), and modern National Elevation Data
set topographic data. A summary of results is
shown in Table 2, and detailed 40Ar/39Ar methods, data, and results can be found in the Supplemental File1.
Elkhead Mountains Region
The Elkhead Mountains represent a significant area of late Tertiary volcanism and
comprise high topography near the ColoradoWyoming border (Fig. 4). The northern flanks
1
Supplemental File. 40Ar/39Ar analytical methods
and results. If you are viewing the PDF of this paper
or reading it offline, please visit http://dx.doi.org/10
.1130/GES00989.S1 or the full-text article on www
.gsapubs.org to view the Supplemental File.

of the range are drained primarily by the Little
Snake River, whereas the southern portions of
the range lie within the Yampa River watershed.
Late Tertiary volcanics of the Elkhead Mountains intrude and overlie the Browns Park Formation (Buffler, 2003) and form elevated mesas
ideal for reconstructing the amount of postincision relief. Of importance to this study, late
Cenozoic extensional faulting is documented
in the region, and displacement across grabenbounding faults (Fig. 6) may be on the order of
~300–600 m (Buffler, 1967).
Battle Mountain, Squaw Mountain, and
Bible Back Mountain. Basalt flows cap the
Browns Park Formation in three locations north
and south of the Little Snake River (Fig. 6).
Atop Battle Mountain, the basal contact of

646

Battle Mtn Basalt Flow
~2680 m
Browns Park Fm

Little Snake River

~650 m

Mesa Verde Group
~2030 m

Geosphere, August 2014

Figure 5. Field relationships
between basalt flows, the Browns
Park Formation, and the Little
Snake River at Battle Mountain,
Wyoming, in the Elkhead Mountains (photo: Russell Rosenberg).
Basalt flows capping the Browns
Park Formation provide an estimate of local relief generated
during late Cenozoic incision.

these flows is exposed in a recent landside;
the underlying Tertiary strata contain two thin,
~0.5-m-thick, tuffaceous layers. The elevation
of the flow base is ~2680 m and stands ~650 m
above the elevation of the Little Snake River. We
determined a 40Ar/39Ar age of 11.46 ± 0.04 Ma
of the basalt flow, which is consistent with the
older K-Ar age of ca. 11 Ma (Buffler, 2003).
Squaw Mountain sits directly across the
Little Snake River southeast of Battle Mountain (Fig. 6). Here, basalts also cap the mesa,
but their base is not exposed, complicating the
interpretation of whether these outcrops represent extrusive flows. Outcrops are non-vesiculated and free of significant phenocrysts, and
evidence for an intrusive or extrusive origin is
equivocal. However, exposed just below the base
of the outcrop are deposits of a volcanic breccia
that is typically associated with extrusive flows
elsewhere in the region (Buffler, 1967). These
volcanic breccia deposits suggest a local surface
vent, and we follow Buffler (1967) in considering the deposits atop Squaw Mountain as extrusive. The exposed thickness of the probable flow
atop Squaw Mountain is ~20 m. We obtained a
new 40Ar/39Ar age on the lowest exposure found
of 11.45 ± 0.04 Ma, which overlaps in age with
the age of the flow at Battle Mountain. The lowest exposure is at an elevation of ~2550 m and
sits ~520 m above the modern elevation of the
Little Snake River.
Overall, the basalt flows at Battle Mountain
and Squaw Mountain lie directly across the
Little Snake River from one another (Fig. 6), are
of essentially identical age, and are at broadly
similar elevations. The relationship of these
two basalt flows to the Little Snake River thus
provides an opportunity for estimating the magnitude of fluvial incision along the Little Snake
directly. Here, we assume that the ca. 11.5 Ma
land surface extended between Battle Mountain
and Squaw Mountain. Taking the average elevation of the two flow bases, ~2600 m, above the
modern elevation of the Little Snake, ~2030 m,
yields an estimate of fluvial incision of ~580 m
since ca. 11.5 Ma. This direct reconstruction of
fluvial incision is similar to the exposed thickness of Browns Park Formation along the Little
Snake and Yampa rivers.
At Bible Back Mountain (Fig. 6), the base of a
~10-m-thick, columnar-jointed flow is exposed
on the southern flank of the peak. Here, it appears
that there may be a second flow of similar thickness above this outcrop, but the nature of the
exposure made this upper outcrop inaccessible.
We obtained a new 40Ar/39Ar age of the basal
flow outcrop of 11.46 ± 0.04 Ma (Table 2). The
elevation of the flow base is ~2550 m and sits
~550 m above the modern Little Snake River.
Map relations suggest that volcanic material is

present at lower elevation toward the northwest,
as mapped by Buffler (1967); these deposits are
discontinuous remnants and probably represent
debris downslope of the unit. The similarity of
the amount of incision (~550 m) to that determined between Squaw and Battle mountains
above lends confidence that this is a relatively
robust measure of the amount of relief generated
during Miocene–Pliocene incision.
Black Mountain and Mount Welba. Geologic relationships between basalt flows in
the southwestern Elkhead Mountains (Fig. 6)
show a markedly different relationship between
the local thickness of Browns Park Formation
and their elevation above the modern river. At

648

Black Mountain, extensive deposits of vesiculated, basaltic debris cover the area adjacent to
and directly below the mesa-shaped peak, but
exposures are rare, and the base of the flow (or
sequence of flows) is not exposed. We sampled
an outcrop on the northeast end of the main
ridge and determined a 40Ar/39Ar age of 10.92 ±
0.16 Ma (Table 2), similar to ages from the eastern Elkhead Mountains presented above. The
lowest exposure of the flow is at an elevation
of ~3160 m.
Nearby at Mount Welba (Fig. 6), exposures
are also poor and difficult to access. There are
three topographic peaks in the vicinity of Mount
Welba. Outcrops of volcanic deposits on the

Geosphere, August 2014

southernmost point, Mount Oliphant, do not display definitive flow textures. However, at Mount
Welba itself, we discovered outcrops of weathered, vesiculated basalt inferred to represent an
upper flow surface. A sample from this exposure
yielded a new 40Ar/39Ar age of 12.60 ± 0.06 Ma
(Table 2). The lowest exposure of the flow is at
an elevation of ~3150 m.
The flows at Black Mountain and Mount
Welba are ~500 m higher in elevation than
Battle Mountain yet sit atop a slightly thinner
section of Browns Park Formation. If we project
these elevations to the main valley of the Little
Snake River, this would predict ~1170–1180 m
of relief, far in excess of the ~350–400 m thick-

Miocene evolution of Rocky Mountain topography
ness of Browns Park Formation exposed at
these localities (Fig. 6). However, the flows at
Black Mountain and Mount Welba sit in the
footwall block of a NW-trending normal fault
system (Fig. 6), and the possibility of syn- or
postdepositional displacement along this structure (Buffler, 1967, 2003) makes projection
to the Little Snake River subject to significant
uncertainty. Rather, we take a more conservative
approach of projecting to the nearest tributary
within the same fault block, Slater Creek and
Elkhead Creek, respectively (Fig. 6); both with
headwater elevations at ~2500 m. This yields
local estimates of incision that are 660 m and
650 m from Black Mountain and Mount Welba,
respectively. The similarity of these values to
the exposed vertical thickness of Browns Park
Formation suggests these are a likely measure of
relief generation during fluvial incision.
Sand Mountain. A thick (&gt;500 m) section of
Browns Park Formation is mapped in the southeastern Elkhead Mountains (Snyder, 1980). The
upper ~300 m of the formation is well exposed in
a landslide scar along the eastern flank of Sand
Mountain (Fig. 6). Here, a sequence of tuffaceous deposits was dated by (Snyder, 1980);
ages range from ca. 12 Ma near the base of the
section to 9.2 ± 1.7 Ma at the top. The section is
capped by andesitic deposits that form the mesalike summit of Sand Mountain proper; portions
of these deposits have been alternatively interpreted as extrusive (Buffler, 1967) and intrusive
(Snyder, 1980).
We re-evaluated these relationships along the
eastern flanks of Sand Mountain and observed
local relationships that support both interpretations. Beneath the summit, andesite is found
at similar elevations to horizontal strata of the
upper Browns Park Formation on either sides of
a steep gully, suggestive of a subvertical, intrusive contact. But, we also discovered outcrops
of porphyritic andesite with weak flow banding that overlie the section on the northeastern
shoulder of the peak. These relationships lead
us to conclude that the andesite is likely a shallow intrusion that has extrusive facies along the
flanks of Sand Mountain. We dated a population
of 15 individual sanidine crystals concentrated
from a sample of the extrusive facies. These
exhibited individual ages ranging from ca. 28 to
9 Ma (see Supplemental File [see footnote 1]).
The youngest three samples cluster around
9 Ma; a weighted mean from these three crystals is 9.05 ± 0.04 Ma (Supplemental Fig. 5 [see
footnote 1]). We consider this a best estimate for
the age of the volcanic deposit because the older
crystals were likely xenocrystic and entrained
during emplacement and/or flow of the andesite.
This age places a minimum bound on the age
of the Browns Park Formation at Sand Moun-

tain. Our results are consistent with the older
fission-track age of the uppermost tephra in the
deposit (9.2 ± 1.7 Ma; Snyder, 1980) but provide
a more precise age. Notably, the Browns Park
Formation must have been present for the intrusive relationships described above. However, we
consider it likely that parts of the andesite were
extruded on top of the Tertiary strata, and, thus,
that the present exposures of the Browns Park
Formation represent most of the pre-incision
thickness. Locally, these inferences imply that
fluvial incision and erosion into the Sand Wash
basin did not begin until sometime subsequent
to ca. 9 Ma. The exposed thickness of Browns
Park sediments in the region implies that exhumation of material from this portion of the Sand
Wash basin was at least 500–600 m, consistent
with our estimates of incision from other parts
of the Elkhead Mountains.
Flat Tops Region
Near the headwaters of the Yampa and White
rivers (Fig. 4), a laterally expansive sequence
of at least 27 stacked basalt flows make up the
large, high-elevation mesas for which the Flat
Tops Range is named (Larson et al., 1975).
Here, basalt flows comprise an overall thickness of ~470 m and range in age from ca. 24 to
9.6 Ma (Larson et al., 1975; Kunk et al., 2002).
Individual flows range in thickness from 3 m to
~60 m where locally ponded against paleotopography (Larson et al., 1975). In the southwest of
the range, most of the stratigraphy is composed
of superposed flows, which become increasingly
intercalated with the Browns Park Formation
toward the northeast (Fig. 7), in the direction
of the Yampa River valley and the Park Range
(Fig. 4). Overall, the sequence of stacked basalt
flows is relatively conformable and lies within
several hundred meters elevation from one
another, despite the wide range in age from ca. 24
to 10 Ma (Larson et al., 1975). This relationship
suggests that basalts were likely extruded onto a
low-relief surface that persisted in the Flat Tops
region until ca. 10 Ma. Thus, we follow Larson
et al. (1975) in inferring that present-day canyons
that dissect formerly continuous flows provide a
measure of incision subsequent to that time.
We estimate the amount of fluvial incision
in the uppermost headwaters of the Yampa and
White rivers by averaging the highest elevation
of the basalt surface on both sides of the modern
valley and subtracting the elevation of the modern river channel. Across most of the Flat Tops
region, the highest interfluves are capped by ca.
20 Ma basalt flows (Larson et al., 1975), but a few
mapped flows that cap the highest peaks (Derby
Peak, W Mountain, and Sugarloaf Mountain;
Fig. 7) range from ca. 15 Ma to as young as 9.6 ±
0.5 Ma (Larson et al., 1975). Although the for-

Geosphere, August 2014

mer extent of all of these flows is uncertain, their
presence on the flanks of the volcanic pile that
comprises the Flat Tops (Fig. 7) suggests that the
present-day relief must have developed subsequent to their deposition. Thus, we consider ca.
10 Ma as a reasonable bound on the timing of
local relief generation in the upper tributaries of
the White and Yampa rivers.
In the headwaters of the White River (A–A′,
Fig. 7) from Lost Lakes Peak to Sable Point, it
appears that there has been ~900 m of fluvial
incision in the past 9.6 ± 0.5 Ma. In the headwaters of the Yampa River (B–B′, Fig. 7) from
Orno Peak to Flat Top Mountain, the magnitude of incision appears to be somewhat less,
~700 m, but still greater than observed in the
Elkhead Range.
Yampa River Valley
The third region we studied is in the headwaters of the Yampa River, north and east of
the Flat Tops Range (Fig. 4). Near the town
of Yampa, Colorado, the river flows north in a
fault- bounded valley before making a series of
sharp bends; east toward Woodchuck Mountain,
north parallel to the flank of the Park Range
(Fig. 8), and eventually west at Steamboat
Springs, Colorado (Fig. 4). Along much of its
course, the river flows in Cretaceous Mancos
Shale and the overlying Browns Park Formation, both of which have been intruded by young
dikes and volcanic plugs (e.g., Kucera, 1962).
Lone Spring Butte. In the western half graben,
a ~10-m-thick, porphyritic, flat-lying basalt flow
with moderately well-developed flow banding is
exposed atop Lone Spring Butte (Fig. 8). In hand
sample, the basalt has phenocrysts of olivine,
plagioclase, and mafic accessory minerals. The
base of the flow is at an elevation of ~3090 m,
~640 m above the modern Yampa River. This
flow unconformably overlies gently dipping,
coarse boulder conglomerates of the basal
Browns Park Formation. Boulders up to ~1 m
in diameter are composed of crystalline gneisses
and granites, similar to those exposed in the Park
Range east of the valley (Fig. 8; Kucera, 1962).
Bedding within the deposit dips ~20°–25° west
and appears to have been tilted in the footwall of
an east-dipping normal fault, which defines the
Yampa Valley half graben (Fig. 8). Volcanic ash
from a thin Browns Park deposit overlying the
basal conglomerates has a zircon fission-track
age of 23.5 ± 2.5 Ma (Izett, 1975; Luft, 1985),
confirming that the underlying conglomerate
represents the base of the formation.
Deposits of volcanic breccia, previously
described by Kucera (1962) and Buffler (1967),
are also exposed along the flank of Lone Spring
Butte, ~300–400 m below the base of the basalt
flow. Similar deposits are present locally through-

649

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